The present invention relates to brushless motor driving circuits, and more particularly to a driving circuit for a two-phase brushless motor having a pair of exciting coils.
A brushless motor operates in such a manner that a rotational angle position of a magnetized rotor is detected by magnetically sensitive elements, such as Hall elements, disposed adjacent the rotor, and in response to the detected outputs of the magnetically sensitive elements, coils are excited to thereby apply rotational power to the rotor. A two-phase brushless motor has a pair of coils disposed in the rotational direction of the rotor with about 90 degrees displacement from one to the other.
A prior art driving circuit for a two-phase brushless motor will be described with reference to FIG. 1. Transistors Q.sub.11 and Q.sub.12 and transistors Q.sub.13 and Q.sub.14 are provided in differential pairs. One output terminal of a Hall element H1 is connected to the bases of the transistors Q.sub.11 and Q.sub.13 while the other output terminal thereof is connected to the bases of the transistors Q.sub.12 and Q.sub.14. A predetermined bias voltage is applied to a bias terminal (not shown) of the Hall element H1. In accordance with the voltage variations between the output terminals of the Hall element H1, the current distribution for the differential pairs is changed. Transistors Q.sub.21 and Q.sub.22 and transistors Q.sub.23 and Q.sub.24 are further provided in differential pairs. The current distribution for these two differential pairs is also changed in accordance with the voltage variation between the output terminals of a Hall element H2. A current sink for the currents of the respective differential pairs includes transistors Q.sub.1 through Q.sub.4, which are biased by a predetermined voltage E.sub.1, and resistors R.sub.1 through R.sub.4 connected to the emitters of the transistors Q.sub.1 through Q.sub.4, respectively. A current supplying source for supplying currents to the respective differential pairs includes a power supply source +B and resistors R.sub.5 through R.sub.8.
The changes of the current distributions for the respective differential pairs produce corresponding changes in the voltage drops across the resistors R.sub.5 through R.sub.8. The voltage variations of the resistors R.sub.5 through R.sub.8 are amplified by operational amplifiers OP.sub.1 through OP.sub.4, the outputs of the operational amplifiers OP.sub.1 and OP.sub.2 are applied to the coil L.sub.1, and the outputs of the operational amplifiers OP.sub.3 and OP.sub.4 are applied to the coil L.sub.2.
In the circuit of FIG. 1, any variations in the characteristic resistance values of the resistors R.sub.1 through R.sub.4 cause variations in the amount of current sunk by the transistors Q.sub.1 through Q.sub.4. This in turn causes fluctuation in the rotational speed of the motor.